1. Field of the Invention
The present invention relates to an endless hot rolling method for continuously rolling sequentially-jointed rolling materials (the "rolling materials" are referred to hereinunder as "rolling materials" for pieces before and during rolling, and "steel strip" for pieces after finishing rolling), the rolling materials differing from each other in any of width, thickness or steel type. More particularly, the invention relates to an endless finishing hot-rolling method which provides an appropriate amount of crown for each steel strip so as to substantially avoid forming inconsistent portions.
2. Description of the Related Art
To achieve thickness consistency over the width of a steel strip by controlling crown, the roll bending method has been widely employed. However, employment of this method alone is insufficient to control crown precisely enough to achieve a currently required level of thickness precision which has become ever-increasingly higher.
To enhance the crown control, Japanese Patent Publication No. 56-20081 discloses a rolling mill which axially shifts a roll having a tapered end portion, and Japanese Patent Application Laid-open No. 56-30014 discloses a so-called CVC shift mill which relatively shifts upper and lower rolls having wave-shape crowns. Such roll shifting method is now used together with the roll bending method to achieve precise crown control.
A known six-high finishing hot-rolling mill employing both the roll shifting method and the roll bending method will be described with reference to FIGS. 5 and 6.
The mill comprises a pair of work rolls 2 for applying rolling load to a rolling material 1, a pair of intermediate rolls 3 each having a tapered end portion, and a pair of back-up rolls 4. The work rolls 2 are vertically movable by means of roll benders 9 which are operated by hydraulic cylinders. The intermediate rolls 3 are axially movable by means of racks and pinions (not shown in figures).
To consecutively roll two rolling materials differing from each other in any of width, thickness or steel type, the above-described rolling mill suspends the rolling operation after completing rolling the preceding (first) material, shifts in an axial direction the intermediate rolls 3 to designated positions, and then rolls while changing the load for bending the work rolls 2 to control the crown shape of the succeeding (second) material.
Lately, so-called endless hot-rolling is employed to enhance the efficiency of hot finishing tandem rolling as described above. In endless hot-rolling, a mill continuously rolls materials different in width, thickness and steel type, after the rolling materials have been sequentially joined together.
To achieve desired crowns in endless hot-rolling, the roll bending load and the roll shift position must be changed in accordance with the dimensions and steel types of the steel strip. However, although the roll bending load can be changed quickly and highly responsively owing to the hydraulic control of a roll bender, the shifting rate of the roll position is very slow. Therefore, when the roll is shifted in the axial direction, particularly, at a joint portion of rolling materials, the crown thereof substantially deviates from a desired crown, thus forming inconsistent portions in the steel strip.
To avoid forming such inconsistent portions, Japanese Patent Application Laid-open No. 62-3818 discloses an improved method for continuously rolling material having different plate widths, the rolling materials having been joined together before rolling. This method comprises the steps of: measuring the width of rolling materials adjacent to a joint portion; shifting the roll position in the axial direction in accordance with the width thus measured, or more specifically, to the difference between the width measured adjacent to the joint portion and a width of other portions of rolling material; and then rolling while changing the roll bending load so as to constantly achieve a desired crown.
However, this method has problems related to the roll shift range occurring during transition from a preceding material to a succeeding material. By this method, the rolls are sometimes shifted out of a desired roll shift range. If this happens, this method undergoes the problems discussed above, that is, many inconsistent portions are formed adjacent to joint portions of steel strips.